1. Field of the Invention
This invention relates to a system for detecting and determining the range of a target vehicle from an automotive vehicle in which the system operates and, more particularly, to a target vehicle detecting and range determining system which receives a signal reflected from a target vehicle located in front of the automotive vehicle equipped with such a system.
2. Description of Related Art
A hazard detecting system for an automotive vehicle which detects hazards and obstructions, such as a leading automotive vehicle which travels ahead of a vehicle including such a system, is well known in the art. Such a system typically sets off an alarm after judging a received signal wave to be a hazard detecting signal wave. Conventionally, the received signal wave is determined to be such a hazard detecting signal wave only when outputs of two sets of received waves, such as supersonic waves reflected by a hazard, are identical. Such a hazard detecting system is known from, for instance, Japanese Unexamined Patent Publication No. 58-131,575.
Using such a hazard detecting system in an automotive vehicle to avoid front-end collisions or accidents on a highway is obviously highly desirable; accordingly, a variety of vehicle distance detecting apparatuses have recently been developed. One such apparatus compares a minimum allowed or critical vehicle distance, estimated in accordance with vehicle velocity, with an actual distance between an automotive vehicle and a subject or target vehicle travelling ahead of the automotive vehicle. A warning is provided to the driver of the automotive vehicle by, for example, an alarm light or a warning sound if the actual distance is less than the critical vehicle distance. Such a hazard detecting system is known from, for instance, Japanese Unexamined Patent Publication No. 57-155,700.
Such vehicle distance detecting systems, however, lack the ability to detect the actual distance between vehicles accurately when the frequency of a transmitted radar wave is the same for two vehicles running in opposite directions around a bend. The systems are unable, in this situation, to discriminate between a radar wave which is reflected from a target vehicle and a radar wave which is transmitted directly from the target vehicle.
To solve this problem, appropriately polarized radar waves can be used. Such enables discrimination of the radar waves of one vehicle from those of another vehicle simply by determining differences in the directions of polarization between the waves transmitted from the vehicles. However, systems employing such polarized waves typically are extremely expensive because the equipment necessary to effect polarization of the radar waves is quite expensive.
Another problem typically found in hazard detecting systems is degradation of traceability of a target vehicle on a winding, a curved, or a steeply inclined road. More specifically, in systems which employ a laser radar to determine the range or measure the distance between vehicles, the output of a laser radar is limited to a certain range to avoid a harmful influence on human bodies. In these systems, the traceability of a target vehicle is improved by controlling the divergence of the laser beam. By way of example, a laser beam of a conventional hazard detecting system could form a spot which is approximately 3.5 meters in diameter at a distance of approximately 100 meters from the laser radar. Degradation of target traceability typically occurs on winding or steeply inclined roads because the spot formed by the laser beam is too small in diameter. If the spot formed by the laser beam is in fact too small, the target will pass out of the spot when it runs on a winding or steeply inclined road, even if the target could be tracked easily on a straight road by the laser beam.